Lithium ion secondary batteries having a high energy density are extensively used in recent years as power sources for small portable electronic appliances such as cell phones and notebook type personal computers. Such a lithium ion secondary battery is produced through the steps of stacking or winding sheet-form positive and negative electrodes together with, for example, a porous polyolefin resin film, introducing the resultant stack into a battery container constituted of, for example, a metallic can, subsequently pouring an electrolyte solution into the battery container, and tightly sealing the opening of the battery container.
Recently, however, such small portable electronic appliances are exceedingly strongly desired to be further reduced in size and weight. Under these circumstances, lithium ion secondary batteries also are desired to be further reduced in thickness and weight. Battery containers of the laminated-film type have also come to be used in place of conventional metallic can cases.
Compared with the conventional metallic can cases, battery containers of the laminated-film type have a drawback that an areal pressure for keeping the distance between the positive and negative electrodes constant cannot be sufficiently applied to electrode surfaces. Because of this, these battery containers has a problem that the distance between the electrodes partly increases with the lapse of time due to the expansion/contraction of the electrode active materials during battery charge/discharge, resulting in an increase in the internal resistance of the battery and hence in a decrease in battery characteristics. In addition, there is a problem that unevenness of resistance occurs in the battery and this also reduces battery characteristics. In the case of producing a sheet-form battery having a large area, there has been a problem that the distance between the electrodes cannot be kept constant and the internal resistance of the battery becomes uneven, making it impossible to obtain sufficient battery characteristics.
In order to overcome such problems, it has been proposed to adhere electrodes to a separator with an adhesive resin layer including an electrolyte-solution phase, a polymer gel layer containing the electrolyte solution, and a solid polymer phase (see, for example, patent document 1).
Furthermore, a method has been proposed which includes coating a separator with a binder resin solution containing a poly(vinylidene fluoride) resin as a main component, stacking electrodes on the coated separator, drying the binder resin solution to form an electrode stack, introducing the electrode stack into a battery container, and then pouring an electrolyte solution into the battery container to obtain a battery in which the separator has been adhered to the electrodes (see, for example, patent document 2).
It has also been proposed to obtain a battery containing electrodes adhered to a separator, by adhering a separator impregnated with an electrolyte solution to positive and negative electrodes with a porous adhesive resin layer to bring the separator into contact with the electrodes and cause the adhesive resin layer to hold the electrolyte solution in the through-holes thereof (see, for example, patent document 3).
However, those processes have had the following problem. The thickness of the adhesive resin layer must be increased in order to obtain sufficient adhesive force between the separator and each electrode. Because of this and because the amount of the electrolyte solution relative to that of the adhesive resin cannot be increased, the obtained battery has increased internal resistance. Consequently, sufficient cycle characteristics and sufficient high-rate discharge characteristics cannot be obtained.
Patent Document 1: JP-A-10-177865
Patent Document 2: JP-A-10-189054
Patent Document 3: JP-A-10-172606